Autonomic nervous system activity is essential for maintaining circulatory and metabolic hemostasis. In order to study sympathetic nervous system function and its relationship to other neuroendocrine systems, it is necessary to measure neurotrasmitter, hormonal, and peptide levels in response to various stimuli. The levels of norepinephrine, epinephrine, and dopamine and their metablolites in various body fluids reflect the activity of the neurones from which these neurotransmitters are released. Although plasma levels of norepinephine reflect the responses of the peripheral sympathetic nervous system it is necessary to consider removal rates of the catecholamine. Measurement of uniary catechlamine metabolites and their stereospecific labelling pattern following administration of radiolabelled isomers of norepinephrine provides a means for investigating intraneouonal norepinephrine metabolism. Cerebrospinal fluid levels of monoamine metabolites can be used to assess central nervous system neurotransmitter metabolism. It is necessary to consider the origin of these metabolites to make appropriate corrections for valid interpretations of the daa. These strategies have been used to study patients with neurogenic orthostatic hypotension and in other cinical situations in which adrenergic function is abnormal. Investigation of the effects of aging on autonomic nervous system function is in progress. A more thorough understanding of neurotransmitter metabolism in these clinical situations leads to more rational approaches to therapy.